The basic principle of such strapping machines from the prior art in various embodiments is based on positioning a binding head or a strapping or closure head on a package that is to be strapped, to guide strapping around the package, to tighten or cinch the strapping and to connect its ends one behind the other inside the binding head. The overlapping ends of the prestressed strap are connected to one another by adhesive force or in another positive manner. The strapping pulled off a supply reel is cut and the binding head is lifted from the package, these steps all being carried out in an automated manner.
A strapping machine of this type with a cincher and closure device of the type mentioned above is known from EP 10194627 [U.S. Pat. No. 4,689,938]. The slide plate that can be moved with the counter electrode like a carriage in the plane of the overlapping region below the welding electrodes is formed by an upper plate and a lower plate that slide against one another or over one another. The upper plate is thus guided on the lower slide plate in which the counter electrode is embedded. With the electrodes pressing the strap ends against one another, an electric circuit initiating the welding operation is closed. This happens when the upper plate has been moved so far away that it exposes the counter electrode embedded in the lower plate, with the strapping lying immediately above the counter electrode and below the advanceable welding electrode. After the welding operation, the upper plate then carrying the lower plate along simultaneously is shifted so far that the welded loop of strapping is released and exposed.
During transport of the packages, for example coiled rod-like or tubular metal products or metal strips wound to form coils, the strapping is subject to very great, sometimes dynamic stresses that it has to absorb. Depending on the material quality and material properties of the packages, one to three straps are placed around the outer surface of the packages to secure it against bursting open and/or to prevent displacement. In any case the strapping must be strongly prestressed, close to the limits of its tensile strength and the breaking strength of the material of the strapping, which can lead to strap impressions on the package.
These problems are further intensified by the counter electrode embedded in the steel slider according to the prior art. The slide plate guided in a groove of a mounting plate must be fairly large, in particular in terms of thickness, in order to be able to absorb the considerable forces employed during the welding operation due to the pressure of the welding electrodes. Since the welded strap after cutting and drawing out the steel slide plate with the embedded counter electrode in the closure region or overlapping region according to the thickness of the slide plate includes the additional height due to the groove guidance snaps back to the outer surface of the package, the strap bearing against the surface of the package in a circumferentially closed manner must be additionally prestressed to compensate for this.